Stimulation of the motor cortex is currently a promising therapy to treat deafferentation pain syndromes, including but not limited to central deafferentation pain arising from stroke, infection, trauma, spinal cord injury, multiple sclerosis, and peripheral pain syndromes including but not limited to trigeminal neuralgia, atypical facial pain, pain arising from peripheral nerve injury or disease including but not limited to nerve plexus avulsion, phantom limb pain, etc. Furthermore, stimulation of the motor cortex or other areas of the cerebral cortex of the brain are potentially promising therapies for treatment of other neurological disorders including, but not limited to movement disorders, neurodegenerative disorders, psychological disorders, and epilepsy, and other central and peripheral disorders.
To date, the best results of chronic motor cortex stimulation for pain have been obtained when stimulation was applied precisely to the zone of motor cortex corresponding to the part of the body in which the pain is experienced. It is therefore essential to respect the somatotopic organization of the motor cortex. The combination of imaging techniques including but not limited to Computerized Axial Tomography (CAT) scans, Magnetic Resonance Imaging (MRI), and three-dimensional neuronavigation procedures for anatomical localization and intraoperative anatomical and electrophysiological testing can greatly improve localization of the zone to be stimulated, thereby improving the clinical results. The objective is to ensure that at least one of the active electrodes is directly over the desired zone of stimulation.
The motor cortex is a narrow band of cortex situated in the precentral gyrus immediately anterior to the central sulcus. The Resume® lead (four electrodes arranged in linear fashion) manufactured by Medtronic, Inc., has been used for chronic stimulation of the motor cortex. If a Resume® lead is placed parallel to the central sulcus, several adjacent zones of the motor cortex can be stimulated simultaneously allowing the treatment of extensive pain, for example involving the entire hemibody. However, as the motor cortex is fairly narrow in the antero-posterior direction and follows the sometimes pronounced convolutions of the central sulcus, the electrode may be inadvertently placed anteriorly or posteriorly to the desired location. It is therefore preferred to place the Resume® lead perpendicularly to the central sulcus in order to ensure at least one of the four contacts of the electrode is directly over the motor cortex. The essential difficulty is to very precisely locate the “height” or mediolateral zone of representation of the part of the body affected by the pain. This localization requires the use of several modalities: imaging data and intra-operative somaesthetic evoked potentials (SEP), and clinical results (muscle contractions) of motor cortex stimulation. Even with these techniques, the limited coverage of this electrode makes it difficult to treat pain involving larger or more extensive regions of the body.
In addition, the method of localization and then implantation of the chronic Resume lead involved two major steps. First, the localization using the modalities mentioned above was performed including the use of a temporary grid of multiple electrodes such as those made by Ad-Tech Medical Instrument Corporation. Once the localization step is completed, the temporary grid electrodes are removed and replaced with a Resume® lead. (In some cases two Resume® leads have been implanted for chronic motor cortex stimulation.) This removal and replacement step adds a layer of complexity and risk of error to the surgical procedure because it requires very precise placement of the chronic lead following removal of the temporary multi-electrode grid used for localization. This step also increases potential short and long-term risks to the patient and extends procedure time. Moreover, financial expense is incurred by the need for both a temporary lead and a chronic or permanent lead.
The location of the paddle when stimulating the motor cortex or elsewhere on the cerebral cortex is beneath the cranium while the lead body is outside of, and must pass through, the cranium. This transition zone over the thickness of the cranium results in mechanical strain between the paddle and the lead body. This strain makes it more difficult to get and keep the entire lead paddle in contact with the tissue being stimulated, and creates risk to the integrity of the lead insulation and conductors.